Fault Current Limiters (FCL) are expected to be among the first and most important power applications of High Temperature Superconductors (HTS). The advantages of HTS-FCL as compared to conventional current limiting devices, used world-wide in national electricity circuits, are their quick response and fast recovery, relatively low energy dissipation, tolerance to large fault currents and the possibility for virtually unlimited number of operations.
More particularly, the present invention relates to current limiting devices based on a superconducting coil with saturated core. In known designs, such a device comprises at least two coils with ferromagnetic cores for each phase connected in series with a load. On the cores there are superconducting bias coils connected to a DC power supply. At normal state the bias coils saturate the cores, and the impedance of the current limiter is very low. When a fault occurs, the current sharply increases and the cores are driven out of saturation at alternate half-cycles. As a result the impedance of the current limiter builds up and limits the current increase.
Two main designs of a saturated core reactor for limiting a fault current in electric power system are proposed in U.S. Pat. No. 3,219,918, incorporated herein by reference. One design includes two AC coils placed on two outer legs of an E-core. Another design employs a single AC coil that encompasses two legs belonging to different cores that are saturated in opposite directions. In this patent DC coils made of copper are envisaged.
In U.S. Pat. No. 3,671,810 incorporated herein by reference this principle has been proposed for transient current limiting in electronic circuits. U.S. Pat. No. 4,045,823 incorporated herein by reference to K. C. Parton et al describes a current limiting device for a power alternating current system. The current limiter has for each phase a pair of saturable reactors whose coils are wound in opposite directions relative to superconducting bias coils. U.S. Pat. No. 4,117,524 incorporated herein by reference also to K. C. Parton et al. describes a modified form of current limiter having a screen of conductive material surrounding the bias winding to shield it against the alternating magnetic field. In this patent, one common bias coil is used for two reactors. Raju et al. [1] realized their current limiting device with a superconducting bias coil operating in a liquid helium bath and demonstrate its efficiency. U.S. Pat. No. 4,257,080 (Bartram et al.) incorporated herein by reference describes a further improvement of this current limiting device by placing the common bias coil on the central limbs of three or six cores of a three-phase reactor. In the three mentioned patents additional air-gapped cores are placed in the circuit of the bias coil. These cores are necessary for decreasing alternating current in DC circuit caused by transformer coupling between the AC coils and bias coils.
Several laboratory scale models of saturable core current limiters have been realized with superconducting coils made of high-temperature superconductors (HTS) [2, 3, 4]. These one-phase [2, 3] and three-phase [4] devices were built according the design proposed in the above-mentioned US patents, the contents of all of which are incorporated herein by reference.
The current limiter with saturated core has decisive advantages as compared with other superconducting current limiters:                its current limiting effect is not dependent on transition of the superconducting element to normal state, i.e. superconducting state is maintained all the time and no recovery time is necessary to return to ready state after fault. Moreover, there is no dissipation of energy associated with transition of the super-conducting element to normal state;        the superconducting element is a coil made of standard superconducting wire manufactured on an industrial scale;        the superconducting coil operates in DC mode and is exposed to low AC magnetic fields.        
Known designs of FCL with saturated cores have essential shortcomings that prevent development and realization of this type of FCL. Its weakest points are the large weight and dimensions that are about twice the weight and dimensions of a transformer of the same power [5]. Also, in known FCLs of this type the impedance of the AC coils does not reach its maximum possible value because the bias coils produce magnetic flux in the cores that reduces the impedance of the AC coils. This feature is necessary at normal conditions but has a negative effect at fault conditions. Furthermore, at fault conditions the alternating magnetic field of the AC coils affects the superconducting bias coil, decreasing its critical current. In known designs, a cryostat with bias coils is placed in the window of the core thus increasing its size. The size of the magnetic core is defined mostly by its cross-section, which in turn is determined by the required voltage drop on the FCL during a fault. This voltage is proportional to the product of the cross-section of the core with the number of turns in the AC coil. The number of turns is limited by allowable voltage drop on FCL at normal operation.
In the above-mentioned WO 2004/068670, we propose new designs that address these considerations. First, instead of closed magnetic cores, open cores (rods) are used. The weight of such core is less than of the closed core. Second, an additional feedback coil is used to compensate the magnetic flux of the bias coil at the fault regime thus increasing the impedance of the FCL limiting the fault current. Use of the additional feedback coil changes the properties of FCL in such a way that both AC coils operate at fault regime during both half cycles. It allows the cross-section of the core to be decreased because the required voltage drop on the FCL is distributed between two coils instead of one at each half-cycle as occurs in previous designs.
However, the transformer coupling inherent in known configurations induces an AC voltage on the superconducting DC bias coil thus superimposing an AC current component in the DC circuit. Moreover, the same effect inheres also to the additional DC circuit of the feedback coil. In all state of art designs the bias coil has a number of turns close to the number of turns in the AC coil and thus the voltage on the bias coil has the same order of magnitude as on the AC coil, i.e. the voltage of the grid at the time of fault.
JP2002118956A2 discloses a current limiter that includes a pair of first and second magnetic cores facing each other, a closed magnetic circuit formed of permanent magnets jointed between the first and second magnetic cores, and a coil by winding a conductor around the first and second magnetic cores, through which saturation magnetic fluxes developed by the permanent magnets flow. The first and second magnetic cores, where the directions of saturation magnetic fluxes are opposite to each other, are formed so that magnetization is reversed alternately by current passing through the coil at short-circuit for each half period of the current.
It would therefore be desirable to provide an improved design of FCL having a superconducting bias coil wherein this drawback is addressed without compromising the advantages afforded by the configuration proposed in WO 2004/068670.